Metabolism of progesterone by the human fetoplacental unit is studied with special reference to the reduction of ring A of progesterone. In human placenta, ring A reducing enzymes are apparently absent, but the fetuses seem to have them detectable by production of metabolites in incubation with progesterone-4-C14. We are studying at present in vitro metabolism of progesterone by the different cellular fractions of liver of human fetuses. The rat placentas and fetuses have enzyme systems for ring A reduction. Progesterone is readily metabolized into 5a-pregnan-3, 20- dione, 3a-OH-5a-pregnan-20-one and 3b-OH-5a-pregnan-20-one. These metabolites have been identified by various analytical techniques such as thin-layer chromatography, reverse isotope recrystallization, gas liquid chromatography and mass spectrometry. We have proposed to study temporal changes in the metabolism of progesterone by the placentas and fetuses of rat during different stages of pregnancy in rat.